Influence of ligand modification on the kinetics of the reactions of iron(III) porphyrins with hydrogen peroxide in aqueous solutions

Abstract

The kinetics of the reactions of a series of iron(III) porphyrins with H₂O₂ to form oxidized, peroxidatically active intermediates have been studied, at 25 °C and ionic strength 0.1 mol dm^–3, by measuring the rates of coupled oxidation of hydroquinone (in some experiments ascorbic acid) under conditions where the reactions were of zero order with respect to reductant concentration. Changes in porphyrin structure were found to influence the reactivity of iron(III) porphyrins in two ways: (1) by affecting the degree of dimerization of the iron(III) porphyrin (rate constants for reaction of the monomeric species showed an inverse dependence on [H⁺]; reaction of dimeric species was only observed for the iron(III) porphyrins having high dimerization constants and the rate constants were independent of pH); (2) to a much smaller extent by exerting a direct influence upon the rate constants for reaction of both monomeric and dimeric iron(III) porphyrin species with H₂O₂.